Growth hormone (herein after “GH”) is a hormone that participates in the regulation of mammalian growth and development. Growth hormone, also known as somatotrophin, represents a class of proteinaceous hormones produced and secreted by the somatotropic cells of the anterior pituitary. Secretion of GH is stimulated by the growth hormone releasing hormone (GHRH) from the hypothalamus and suppressed by somatostatin. This pituitary hormone exhibits a number of biological effects including somatogenesis, lactation, activation of macrophages, insulin-like and diabetogenic effects among others (Chawla, R, K. (1983) Ann. Rev. Med. 34, 519; Edwards, C. K. et al. (1988) Science 239, 769; Thorner, M. O., et al. (1988) J. Clin. Invest. 81, 745). Human growth hormone is a member of a family of homologous hormones that include placental lactogens, prolactins, and other genetic and species variants of GH. GH regulates the secretion of insulin-like growth factor (IGF-1, formerly known as somatomedin C), among other peptide hormones known collectively as somatomedins, which account for most of the biological activities.
Species-specific forms of GH have seen wide-spread use in domestic animals. For example, bovine growth hormone (“bGH”), also known as bovine somatotropin (abbreviated bST and BST), is a protein hormone produced in cattle. Recombinant forms of bGH have been utilized to increase the milk yield in lactating cows (Dohoo I R, Can J Vet Res. (2003) 67(4):241-251).
The growth velocity and body composition of humans, farm animals, and companion animals appear to respond to GH replacement therapies under a broad spectrum of conditions. GH directly stimulates division and multiplication of chondrocytes of cartilage, and GH also stimulates production of IGF-1 in canines (Prahalada S., et. al., Horm Metab Res. (1999) 31(2-3):133-137). IGF-1 has growth-stimulating effects on a wide variety of tissues. Additional IGF-1 is generated within target tissues, making it apparently both an endocrine and an autocrine/paracrine hormone. IGF-1 also has stimulatory effects on osteoblast and chondrocyte activity to promote bone growth.
Growth hormone (GH) plays a key role in somatic growth through its effects on the metabolism of proteins, carbohydrates and lipids. In addition to its effects on somatic growth, GH has been shown to stimulate blood cells in vitro (Derfalvi et al., 1998; Merchav et al; 1988), to increase erythrocytes and hemoglobin counts (Valerio et al., 1997; Vihervuori et al., 1996), to enhance both proliferation and Ig production in plasma cell lines (Kimata and Yoshida, 1994) and to stimulate CD8+ cell counts and, to a lesser extent CD4+ cell counts (Geffner, 1997).
A number of diseases and disorders are associated with the deficiency of GH. For example, as mammals age, the growth hormone releasing hormone-GH-insulin growth factor-1 (GHRH-GH-IGF-I) axis undergoes considerable decrement, with reduced GH secretion and IGF-I production, resulting in a loss of skeletal muscle mass (sarcopenia), osteoporosis, increased fat deposition, decreased lean body mass, and/or other disorders. Studies in humans and other mammals have demonstrated that the development of these changes can be offset by recombinant growth hormone (“GH”) therapy.
The 22 kDA molecular weight of GH is well below the threshold value for kidney filtration of about 70 kDa (Caliceti (2003) Adv Drug Deliv Rev 55:1261-1277), which contributes to the serum half-life of native hGH being less than 20 minutes in humans. Thus, commercial preparations of GH must be dosed frequently to achieve benefit. A sustained-release form of human GH, Nutropin Depot (Genentech and Alkermes) was approved by the FDA in 1999, allowing for fewer injections (every 2 or 4 weeks instead of daily); however, the product was discontinued in 2004.
Chemical modifications to a therapeutic protein can reduce its in vivo clearance rate and increase subsequent serum half-life. One example of a common modification is the addition of a polyethylene glycol (PEG) moiety, typically coupled to the protein via an aldehyde or N-hydroxysuccinimide (NHS) group on the PEG reacting with an amine group (e.g. lysine side chain or the N-terminus). However, the conjugation step can result in the formation of heterogeneous product mixtures that need to be separated, leading to significant product loss and complexity of manufacturing and does not result in a completely chemically-uniform product. Also, the pharmacologic function of the therapeutic protein may be hampered if amino acid side chains in the vicinity of its binding site are modified by the PEGylation process. Fusing an Fc domain to the therapeutic protein is another approach to increase the size of the therapeutic protein, hence reducing the rate of clearance through the kidney. Additionally, the Fc domain confers the ability to bind to and be recycled from lysosomes by the FcRn receptor, which results in increased pharmacokinetic half-life. Unfortunately, the Fc domain does not fold efficiently during recombinant expression, and tends to form insoluble precipitates known as inclusion bodies. These inclusion bodies must be solubilized and functional protein must be renatured from the misfolded aggregates. Such process is time-consuming, inefficient, and expensive. Accordingly, there remains a need for improved growth hormone compositions with increased half-life which can be administered less frequently, and/or be produced by a simpler process at a cheaper cost.